Helium occurs in very low concentrations in certain natural gas fields. Natural gas streams from which helium can be economically recovered typically contain approximately 0.1% to 0.5% helium. This helium must be upgraded to produce a crude helium stream containing typically at least 30% helium.
Producing a crude helium stream is usually done in two or more successive upgrading steps. The first upgrading step generally involves the separation of the feed into a helium-lean gas stream comprising the majority of the feed stream and a smaller helium-enriched stream. It is the most power and capital intensive step in the overall process and it also directly impacts the energy and capital demands of downstream processing.
Existing methods for providing a helium-rich stream from a natural gas feed suffer from one of two drawbacks. The simple, low-capital approach produces a helium-rich stream which has a relatively low concentration of helium yet a relatively high flowrate. On the other hand, the alternate processes which produce a helium-rich stream with a higher helium content and lower flowrate require the use of more complicated equipment which results in a higher capital requirement.
Numerous processes are known in the art for the cryogenic separation of helium from a natural gas stream; among these previous attempts to solve this problem are the multi-stage flash cycle and the high pressure stripping process, both of which involve the recovery of the major portion of the helium in a separation performed at feed pressure.
In the flash cycle, which is disclosed in U.S. Pat. No. 3,260,058, feed gas is partially liquefied and phase separated. The helium-enriched vapor thus produced contains about 80% of the helium contained in the feed gas. Dissolved helium in the liquid portion is recovered by several subsequent flash steps in which small amounts of helium-rich vapor are flashed off and eventually added to the bulk helium-rich stream.
The flash cycle has the advantages of simplicity and low capital cost. However, the concentration of helium in the helium-enriched vapor stream is relatively low. For instance, given a natural gas feed stream containing about 0.4% helium, the concentration of helium in the helium-enriched stream is only about 2%. Therefore, the flowrate of the helium-enriched stream is about 20% of the feed gas flowrate. This relatively high flowrate leads to high capital and power costs for subsequent upgrading steps.
In the distillation (high pressure stripping) process, which is disclosed in "A New Approach to Helium Recovery", Kellogram Issue No. 3, M. H.. Kellogg Co; 1963, feed gas is at least partially liquefied and fed to a distillation step in which dissolved helium is stripped from the liquid at feed pressure. The vapor product from the stripping step contains from 97% to 99.5% of the helium contained in the feed stream.
The high pressure distillation process has the advantage of higher helium content in the helium-enriched stream than the flash cycle. For instance, given a natural gas feed stream containing about 0.4% helium, the concentration of helium in the helium-enriched stream is about 2.5% to 3.0%. Therefore, the flowrate of the helium-enriched stream is about 13% to 16% of the feed gas flowrate. In addition, since the helium-enriched stream is produced at feed pressure, the product streams from the subsequent processing steps can be returned at higher pressure, thereby reducing energy consumption for the crude helium stream recompression.
The disadvantage of the high pressure distillation process is high capital cost due to the difficulty of performing a distillative separation at high feed pressure and the complexity of supplying reboil duty to the stripping column. The difficulty of the separation leads to a relatively high reboil duty required for high helium recovery. This high vapor flowrate coupled with unfavorable surface tension and vapor-liquid density difference leads to large column diameters.
Reboil duty is supplied to the stripping column using a methane heat pump, which requires additional energy and heat transfer equipment. The combination of the large column size and the methane heat pump lead to a high capital cost for this process.
U.S. Pat. No. 4,758,258 discloses another process for cyrogenically separating a helium-bearing natural gas stream comprising subjecting the natural gas stream to a sequence of alternating cooling and separating steps. In the process, one or more process-derived streams are utilized to effect cooling of the natural gas stream to temperatures in the cryogenic range. The process provides for the separation and recovery of a natural gas liquids product stream consisting of substantially condensed C.sub.2 and higher hydrocarbons and a gaseous product stream consisting of at least 50 volume percent of helium with the balance being substantially nitrogen.
As is apparent from the above discussion, the prior art is wanting for a simple, efficient, low-cost method of processing a natural gas feed to produce a helium-rich stream with high helium content. The present invention is an answer to that wanting.